As far as I know, the difference is mainly between whether you have to retrieve the results from the server or not.
To get $info['count']
you have to retrieve the complete result from the server via ldap_get_entries
which might be a lengthy thing depending on the size of the result and the connection to your LDAP-Server.
To check whether it's worth the effort you can get the size of the resultset with ldap_count entries
and depending on that result your application can decide what to do.